The present invention relates to a method of optically recording data, and more particularly to a recording method which is well suited to record signals utilizing the rise and decay of pulses for data, on a recording medium such as optical disc.
In an information recording and reproducing system which exploits the recording and reproducing principles of an optical disc, information is recorded by changing the optical or magnetic property of the medium at parts thereof. The recording and reproducing system of this type is described in, for example, `Philips technical review`, Vol. 40, 1982, No. 6, pp. 157-164.
Heretofore, in case of recording a modulation signal utilizing the rise and decay of pulses for data, for example, a signal such as the NRZI code on a recording medium such as optical disc, the intensity of a laser beam has been modulated with the modulation signal itself. That is, a laser output has been switched with the rise and decay of the logic of an input signal code.
In an optical disc system, information is recorded by exploiting the thermal property of a recording medium. Points on the recording medium corresponding to the rise and decay of a recording pulse are the front edge and rear edge parts of a pit or magnetic domain formed by the recording pulse, respectively. As regards the temperatures of the front edge and rear edge parts at the step of forming the pits or magnetic domains, the temperature of the rear edge part becomes higher than that of the front edge part due to the effect of thermal diffusion. For this reason, as the pulse widths of the recording pulses or laser output waveforms increase as shown by waveforms 201, 202, 203 and 204 in FIG. 1, the influence of the effect of thermal diffusion manifests itself, and the pits or magnetic domains become shapes expanded at the rear edge parts as seen from pits or domains 213 and 214. Here, when note is taken of a reproduced waveform 224 obtained from the pit or domain 214, the rising waveform 225 and decaying waveform 226 of the reproduced waveform 224 are not symmetric. When such an asymmetric reproduced waveform 224 is passed through a level slicer with the intention of detecting the recording pulse width, an error develops because the variation 235 of the front edge and that 236 of the rear edge of a detected pulse 234 become unequal with respect to the shift 227 of a slice level. That is, when the recording pluse widths are detected from the asymmetric reproduced waveforms, the probability of occurrence of errors rises, and a high reliability is not attained.
In FIG. 1, numerals 211, 212, 213 and 214 designate the shapes of pits or magnetic domains formed by the recording pulses 201, 202, 203 and 204 respectively, numerals 221, 222, 223 and 224 designate reproduced waveforms obtained from these pits or domains respectively, and numerals 231, 232, 233 and 234 designate pulses detected from these reproduced waveforms respectively.
Meanwhile, in the production of discs dedicated to reproduction such as a compact disc and a laser disc, the photoetching process is employed for forming pits. According to this process, the shapes of the pits are determined by irradiation energy density profiles only. When the front and rear edges of the pits are given symmetric irradiation energy density profiles, they become symmetric without incurring the problem of the asymmetry of the edges of the pits.
In the optical disc system wherein information is recorded by exploiting the thermal property of the recording medium, there has already been proposed a recording method in which the waveform of a recording pulse is converted into a waveform with the effect of thermal diffusion considered in advance, whereupon a reproduced waveform of excellent symmetry is obtained from a pit or magnetic domain formed by the use of the converted waveform. This proposal is described in the official gazette of Japanese Patent Application Laid-open No. 60-25032.
The proposed method is illustrated in FIG. 2. In this method, a current source circuit for driving a laser is furnished with a differentiation circuit which consists of a capacitor C and a resistor R. In response to a rectangular input modulation signal waveform 19, a laser output waveform 20 is prepared the pulse height 21 of which attenuates exponentially in accordance with a time constant C.multidot.R. This waveform 20 is used as a recording pulse. That is, a laser beam intensity at the rear end part of the recording pulse is made lower than that at the front end part of the recording pulse, thereby to establish a uniform temperature profile from the front edge to the rear edge of a pit or magnetic domain. Therefore, the pit or domain 22 of good symmetry is formed, and a reproduced signal waveform 23 of good symmetry is obtained.
With this method, however, the optimum time constant C.multidot.R for the thermal diffusivity of a recording medium (recording film) must be set. In this regard, the time constant C.multidot.R cannot be conformed to a dispersion in the thermal diffusivity of the recording film attributed to the fluctuations of conditions for preparing the recording film. This method therefore has the disadvantage that the pits or magnetic domains of good symmetry cannot be always formed stably.